Optical dense wavelength division multiplexing (DWDM) networks typically use modern iteratively decoded forward error correction (FEC) codecs with hidden error floors at very low bit error rate (BER) levels. When secure transport, featuring authenticated encryption is added to such networks, there is a potential that post FEC-errors lead to erroneous authentication fail (AF) events. These erroneous AF events are a consequence of the FEC-error and not indicative of non-authentic traffic.
Such high-throughput coherent optical transmission systems employ modern FEC codecs with iterative soft decoding in order to allow operating at high levels of pre-FEC BERs. Unlike Reed-Solomon FEC codecs which have a predictable and an increasingly steeper BER “waterfall” curve, these modern codecs typically exhibit an error floor which typically sets in at a low post-FEC BER (typically below a BER of 10−15).
In iteratively decoded FEC codecs, the error floor may be caused by error patterns that ‘confuse’ the decoder. Such error patterns may be referred to as “trapping sets”, or as “stall patterns”, since the decoder appears to “stall”, i.e. get locked without further reduction of bit errors over iterations.
The post-FEC BER actually required for long term error free operation is significantly below what can actually be measured and verified (e.g. below 10−18, further decreasing with increasing throughput). Therefore, because the error floor might actually begin, for example, at 10−16, it may be the case that sporadic post-FEC Errors (called Background Block Errors in older literature and standards) will occur during field operation, when classical system margin allocations are used.